Present methods for determining the color of an object are based on measuring the spectral reflectance of the object as illuminated by a polychromatic light source and by determining the tristimulus values X, Y, Z, or L*, a*, b* for determined illuminants.
However, the results which are obtained are generally not satisfactory when such methods are applied to determining the color of an object that is translucent, as applies to a tooth in the mouth.
In principle, reconstituting a color by means of tristimulus values is not very reliable at wavelengths that correspond to transitions between the primary colors, i.e. in the vicinity of the wavelengths at the intersections between the curves of the distribution coefficients x, y, and z of the CIE colorimetric reference observer: it is at these wavelengths that the human eye perceives the greatest differences of hue for a given variation in wavelength.
In addition, teeth have large hue dispersions in the vicinity of these wavelengths, which provides an explanation for the poor results of known methods. The translucent nature of teeth constitutes an additional source of error since teeth that are apparently very white may have a light reflection coefficient that is half that of teeth that are yellower or browner in hue, thereby modifying the spectral reflectance and giving rise to erroneous conclusions.
An object of the invention is to provide a method and apparatus enabling the color of a tooth to be determined reliably, taking account of its transparency or translucency, in particular for the purpose of making a dental prosthesis having the same color.
More generally, another object of the invention is to provide a method and apparatus enabling the color of a translucent object to be determined without risk of error.